SUMMARY


SUMMARY





The Romanche Fracture Zone is one of the widest and important transform discontinuities of the oceanic plates. It shows a strike-slip displacement of 960 km for a length of about 1000 km as a consequence of within plates tectonism of Cretaceous age.
The oceanic crust was uplifted and tectonized exhibiting a great variety of oceanfloor rocks. In addition the sedimentary rocks, the igneous products range in composition from the ultramafites to the tholeites and comprise gabbros, plagiogranites and volcanites of alkaline affinity.
The eastern part of the Romanche Fracture Zone (between 16ºW55' and 0ºS15'-0ºN42') was investigated during the Equamarge II cruise (R/V Charcot, Dakar-Abidjan Leg) using GPS, Seabeam and SBP 3.5 kHz.
The investigated Fracture Zone can be schematically described as a U-shaped valley (16 km wide) between 5200 and 6500 m water depth, bordered to the North and the South by steep ridges (max 27º slope). The base of the Fracture Zone is relatively flat and orientated N76º-77ºE. In detail, however, the valley floor is characterised by a relief of small scale highs and lows, orientated in the same direction.
The Southern Ridge forms a fairly regular wall (slope 10º-17º) which is sometimes interrupted by deep incisions. The summit of the ridge is covered by sediments with a maximum thickness of 0.4 std. The Northern Ridge forms the southern boundary of the Sierra Leone Abyssal Plain (5300 m water depth): this ridge includes the Pillsbury Ridge (once called Pillsbury Seamount) which reaches a water depth of about 1000 m. About half way down the southern slope of the Northern Ridge is a suspended valley with a slightly different orientation to that of the main valley (N87º). The suspended valley is filled with between 0.4 and 0.8 sdt sediments.
During the last period of the cruise was investigated the fossil (inactive) trace of the Romanche, i.e. the basement cropping out from the sedimentary cover, initiating from the Guinea Gulf offshore to the margin-slope transition of the Ivory Coast-Ghana at 2ºN,6ºW where it is still visible. It is asymmetric with ENE-WSW trend (mean direction N77ºE). Along strike it can be divided in three parts. The western one tops at -3700 m with maximum dip of about 18º (mean direction N77ºE); the middle one shows a step of ab. -4900 m with dip of ab. 9º and the eastern one has shape that seems similar to that of the first segment, but with slightly different orientation (N70ºE). It culminates at -4300 m.





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